This invention relates to noise suppression apparatus for gas turbine engines. More particularly this invention relates to a combined combustor and noise suppressing assembly for reducing combustion noise in a gas turbine engine.
Recent advances in the design of gas turbine engines have not only improved engine performance, but have also greatly reduced engine noise levels. Gas turbine noise sources that were once considered to be of minor importance have become major factors in designing an engine to reduce its noise emissions. Further reductions in gas turbine engine noise levels can still be achieved by substantially reducing the noise output of these noise sources. One such source of jet engine noise, commonly identified as "core noise," emits relatively low frequency noise resulting from the fuel combustion process and turbulent air flow passing over the jet engine combustor components. Once generated, this low frequency noise passes through the engine core, through the engine turbine structure, and out the engine tailpipe.
One proposed technique for suppressing core noise is reported by Kazin et al in American Society of Mechanical Engineers Paper No. 74-WA/Aero-2, entitled "Low Frequency Core Engine Noise." Kazin et al disclose that approximately 10 decibels of noise suppression can be obtained by a cavity resonator located directly behind or downstream from the engine combustor assembly. Although suppression of core noise by 10 decibels is a substantial improvement, application of this technique would generally require major changes in the design of gas turbine engines, since the region downstream of the combustor structure in a conventional gas turbine engine is almost entirely occupied by the engine turbine structure. Further, such an approach relies solely on attenuation of an established noise source rather than modifying the characteristics of the noise-producing process to control the generated noise level.
Noise source modification to control the acoustic wave fronts generated by a combustion process is known in the art and has been applied to eliminate or reduce combustion irregularities and/or undue heating and vibration of gas turbine engine combustor structure. For example, U.S. Pat. No. 2,807,931, issued to Albert G. Bodine, Jr. discloses acoustic attenuators placed directly into the outer combustor wall of a jet engine. These acoustic attenuators are located at points of high acoustic impedance to eliminate or reduce acoustic resonance of the combustor to thereby improve the combustion process by eliminating combustion irregularities or instabilities. Although Bodine seems to recognize that there is a complex functional relationship between the acoustic properties of a gas turbine engine combustor structure and the combustion process, the Bodine disclosure does not address the conceptually related but quite different problem of constructing and arranging an engine combustor to reduce noise levels.
Accordingly it is an object of the this invention to provide a noise suppressing combustor assembly for use in conventionally configured gas turbine engines. It is a further object to this invention to provide a combined gas turbine engine combustor and noise suppressor arrangement that is configured to reduce combustion noise by both the modification of the combustion process and by the attenuation of the noise produced by the modified combustion process.